Got a client with Small Business Server (SBS) 2011 running on Hyper-V Server Core 2008 R2. I always remote in to load updates by using Remote Desktop (or Terminal Services client or Remote Desktop Services or Client or whatever its called this year)
I noticed this particular server always gives an Unexpected Shutdown after I click Restart once the updates are loaded. Also if I manually choose to Restart.
Couldn't see what was causing it until I came across this thread where a load of people with Blackberry Enterprise Server were having the same problem (this is a Dell Poweredge by coincidence too).
Scroll most of the way down for the post from JakesRNC. Its a quick and dirty fix but does the job. Putting 'net stop "Blackberry Controller" in the shutdown script for the server with Group Policy.
On the server,
Start > Run > gpedit.msc
Computer Config > Windows Settings > Scripts > Shutdown
Add a script with
name = net
parameters = stop "Blackberry Controller"